All "wedges" are now fan sprung. In other words the spring elements, whether they be sinuous bands, Pirelli webbing, or any other resilient support elements, are arranged in a fan-like pattern extending between the short, straight, front rail of a wedge frame and the long, arcuate, back rail of the frame (or a plurality of supplementary rail members subtending chords of the arc across its length). In this regard, conventional straight, front rail length is on the order of sixteen to eighteen inches while arcuate, back rail length is on the order of four or more times the front rail length.
Where sinuous springs are employed, for example, as many as nine individual spring bands are employed, and invariably fan sprung. At their converging ends, the connections to the front rail are very close together, often touching. The strain exerted on the front rail by these closely grouped springs is very substantial; so great as to occasionally cause rail splitting. As such, the frame must be very sturdily constructed and, as a result, it costs more to make than conventional frames. Furthermore, the quality of the seat at the front rail is very poor, i.e., it is so stiff as to give the seating quality of a springless cushion support system. In contrast, the seat quality at the back rail is soft as to be mushy and lack support, since fan-sprung diverging springs are as far as ten inches apart there.
In essence, the furniture industry expects conventional "wedges" to sit terribly. Their lack of comfort is accepted in the interest of design requirements in corner groupings of cushioned furniture.